Artificial stereophonic sound system



June 10, 1958 s. w. ATHEY ETAL ARTIFICIAL STEREOPHON IC SOUND SYSTEMFiled Jan. 13, 1955 2 Sheets-Sheet 1 HMPL.

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June 10, 1958 -s. w. ATHEY ETAL 2,838,608

ARTIFICIAL STEREOPHONIC SOUND SYSTEM Filed Jan. 1:5, 1955 f 2Sheets-Sheet 2 IN VEN TOR.

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United States I atent ARTIFICIAL STEREOPHONIC SOUND SYSTEM Skipwith W.Athey, Mount Kisco, and Weld S. Carter, Jr., Peekskill, N. Y., assignorsto General Precision Laboratory Incorporated, a corporation of New YorkApplication January 13, 1955, Serial No. 481,556.

5 Claims. (Cl. 179-1) This invention pertains to sound reproductionsystems which relate the apparent sound source location to the characterof the sound. This invention particularly concerns the generation ofstereophonic sound by the use of only a single electrical sound channel.

The reproduction of moving visual images on a screen,

such as in :motion pictures and in television, is conventionallyaccompanied by sound reproduction. Thus the speech of actors seen on thescreen, or music or sound effects, accompany the action on the screen.When the screen is large, as in theatre motion pictures and theatretelevision, the naturalness of the reproduction is heightened if thesound accompanying an action appears to originate at the location of theactor or other source on the screen. Several loudspeakers are concealedbehind the screen and the sound accompanying an action is caused to be.emitted only bythe loudspeaker concealed behind the appropriate screenlocation.

Inconventional motion picture stereophonic systems a plurality ofmicrophones are placed for reception of sound from selected parts of thestage; The microphone outputs generate separate sound tracks on themotion picture film beside the pictures thereon, and in projection theseparate sound tracks operate individual loudspeakers located atdifferent positions behind the motion picturescreen. Thissystem requiresspecial, film carrying a, plurality of sound tracks.

The instant invention provides a stereophonic effect but employsconventional motion picture film in the sense that. the film carriesonly a single sound track. This system therefore, requires but a singlesound channel. The film employed is unique in that it carries on thesingle sound track, in addition to the sound to be reproduced with thepicture, a plurality of tones which are rendered inaudible to thelistener but which serve as control signals. In making this special filma plurality of alternating signal currents are added to the audiocurrents of the pick-upmicrophone circuit. These signal currents havefrequencies outside of the audio current frequency range,

. and may be either higher or lower in frequency, although as:apractical matter it is convenient to position the signal currents in therange below 50 C. P. S. These control signals are manuallyadded, withoutintermodulation, at such times and in such intensities as to representat 7 several stage locations the instant sound distributions.

For example, a 30 C. P.1S. signal may represent sound emanating from theright of the stage, a 35 C. P. S.

, signal may represent a center stage source and a 40 C. P. S.- signalmay represent a left stage source. Now

'if an actor at the right speaks, only the 30 C. P. S. signal isapplied, at full amplitude. If anactor speaks at left center on thestage perhaps both the 35 C. P. S. and

. the 40 C. P. S.- signals will be introduced, but each at half iamplitude, representing the location of the source between centertstageand left stage.

2,838,608 Patented June 10, 1958 "ice amplified, and is transmitted to acontrol unit, where the audio, signal is separated from the controlsignal. The former is transmitted through three sets of amplifiers tothree loudspeakers positioned behind the screen, one. at the left, oneat the right and one in the center. The control signal containing only30, 35 and 40 C. P. S. frequencies is separated into its three frequencycomponents, and these are made respectively to control the gains of thethree amplifiers. Thus each of the loudspeakers is individuallycontrolled by a control signal as to the amplitude of its output inaccordance with the location of the sound source in the scene originallyphotographed. Each loudspeaker is additionally varied in amplitude inthe usual manner in accordance with the varying amplitudes of thesource.

The instant invention also provides several refinements necessary inconnection with the use of existing projection equipment and present-daymotion picture projection practices. For example, when conventional filmhaving soundtrack carrying no control signal is utilized, the device ofthe invention automatically changes to single-output-channel operation,and when film having a single sound track adapted to the uses of thisinvention is employed the device automatically changes to the virtualstereophonic loudspeaker operation of this invention. This inventionalsoprovides automatic reversion to single-channel operation uponfailure of almost any part of the equipment of the invention. Thissinglechannel or non-stereophonic mode of operation may of course employany number of the described loudspeakers operated as a single unit, butfor clarity the detailed description describes a circuit in which butone speaker is employed. Since in single-channel operation theelectronicv portions of the entire control unit are bypassed and thesignal channel contains only passive equipment, such operation is notsubject to electronic failure and thus is highly reliable. The equipmentalso contains level measuring equipment to facilitate proper settings ofgains to insure stability of operation.

The general purpose of this invention is to provide a device forsynthesizing stereophonic sound signals from a single audio channelcomposite signal.

Another purpose of this invention, in theatre screen reproduction ofimages andaccompanying sound and employing a single electricalsoundchannel, is to generate sounds seeming to emanate from a selectedone or ones of a plurality of positions on the screen.

Still another purpose of this invention is to provide an artificialstereophonic sound system employing only one inputaudio channel withautomatic change-over from single-channel to stereophonic operation andvice versa.

Still another purpose of this invention is to provide an artificialstereophonic sound system employing only a singlev audio input channel,the system automatically reverting to single-output-channel operationupon failure of any part thereof. 7

Still another purpose of this invention is, in motion picturereproduction of images with accompanying sound reproduced from a.singleelectrical sound channel, to generate sounds seeming to emanate from aselected one or ones of a plurality of positions on the motion picturescreen in consonance with the visual action reproduced thereon.

A further understanding of this invention may be I secured from thedetailed description together with the Inprojecting the film, thecomposite sound signal is derived from the film sound track in the soundhead, is

accompanying drawing in which Figures 1 and 2 when taken togetherconstitute a schematic diagram of an in strument embodying theinvention.

Referring now to Fig. 1, a motion picture projector attenuator 22connected in shunt thereto.

I I 3 sound head 11 is energized of a motion picture film. The soundtrack contains a composite sound record composed of the audio signalrecord which is to be reproduced and heard accompanyingthe picturereproduction, and a control signal record. The control signal record isan additive combination or sum of three component control signal recordsand this sum amplitude is designed to be constant. It is actually heldin practice as near a constant value as the art of the operator mixingthe signals during filming can make it. The constant sum of the threecontrol signals is ten percent of the total allowable maximum amplitudeof the sound track record. The audio signal is conventional, andincludes all frequencies between 50 C. P. S. and 12,000, with attenuatedsignal below 50 C. P. S. The control signal is composed of threeconstant-frequency control signals having frequencies of 30,35 and 40 C.P. S. The amplitudes of these three signals have been adjusted in makingthe film in 's uch manner that each amplitude at any instant representsthe presence or absence of sound origination at a selected section ofthe stage being filmed. The 30 C. P. S. signal is chosen to representthe origination of sounds from the right side of the stage, the 35 C. P.S. signal represents the center, and the 40 C. P. S. signal representsthe left side. These three control signals and the audio signals havebeen simply added substantially without intermodulation to make thecomposite sound track record.'

The electrical output of the projector sound head 11 is conductedthrough a single electrical channel 12 to a preamplifier 13 and througha single electrical channel comprising conductors 14 and 14 to terminals16 and 16' where the channel is split into two parallel channels.

' These terminals 16 and 16 are connected through conductors 17 and 17'to a primary winding 18 of a transformer 19, and through conductors 21and 21' to an These two parallel channels constitute the respectivecontrol signal channel and audio signal channel. The audio signalchannel 21, 21 is made more non-receptive to the control signals by theinterposition of a high-pass filter 23, and the control signal channelis made non-receptive to the audio signal by the interposition of alow-pass filter. The attenuator 22 is provided so that the audio signalamplitude can be adjusted Without affecting the control signalamplitude, and takes the place of a ganged fader conventionally used atthe power amplifier inputs. The attenuator 22 is employed, rather thancontrol devices before the splitting terminals 16-16, because of the lowlevel of the control signal and'because the control signal amplitudemust remain under the exclusive control of the input control signalrecords.

Three loudspeakers 26, 27 and 28, Fig. 2, are positioned behind theright, center and left portions of the motion picture screenrespectively, and each is energized by its power amplifiers 29, 31 and32. Power amplifiers 29 and 32 are switched in and out of circuit bytheir respective relay contact armatures 33 and 34, and power amplifier31 is switched between two circuits by its relay contact armature 36. Inthe drawing all relay contact armatures are shown in the normal'positionwith the relay coil unenergized. In this position of the relay theloudspeakers 26 and 28 are out of the circuit because their poweramplifier inputs are short circuited and the center loudspeaker 27 isenergized through normal contact'37, conductor 38, warping circuit 39,conductor 41, normal or back relay. contact 4-2, (Fig. 1) relay contactarmature 43, and conductor 44 connected to the high pass filter 23. Thefunction of the warping circuit 39 is to modify the high end of thefrequency characteristic in accordance with requirements. The centerloudspeaker in this condition of operation is the only loudspeaker inthe circuit, receives all of the audio signal and provides singleoutput-channel operation. It is to be from the single sound track notedthat this single-output-channel operation is provided through a pathcontaining no stereophonic equipment, so that such operation is notinterrupted by any failure of stereophonic components, an essentialfeature for motion picture theatre service continuity.

When the relay armatures 33, 34 and 36, Fig. 2, are

operated to their forward or'upward positions and relay armature 43,Fig. 1, of the same relay is operated to its forward or down position,the three loudspeakers are connected for virtual stereophonic operation.The audio signal is applied from the high pass filter 23 throughconductor 44, armature 43, forward contact 46 and conductor 47 to awarping circuit 48 which modifies the frequency characteristic inaccordance with the possibly diiferent requirements of stereophonicoperation. From the warping circuit 48 the audio signal is appliedthrough an attenuator 49 and transformer 51 to three control amplifiers.Attenuator 49 permits adjustment of the virtual stereophonic channel forapproximately unity gain, so that this channel will have the same gainas the single output channel, which contains no amplifiers. Thetransformer 51 matches the attenuator 49 output impedance to that of thethree control amplifier input circuits in parallel. The groundedcenter-tapped secondary winding 52 of transformer 51 is connectedthrough conductors 53 and 53', Fig. 2, to three pairs of couplingcapacitors 54, 54', 56, 56 and 57, 57' connected to the control grids ofthe three control amplifiers. These amplifiers comprise push-pullpentodes 58, 58', 59, 59', 60 and 60'. Their outputs are connected tothe three power amplifiers through output transformers 61,. 62 and 63and forward relay contacts 64, 65 and 66.

The'control signal, consisting of a mixture of three signals havingfrequencies of 30, 35 and 40 C. P. S., is applied to the primary winding18 of transformer 19. The secondary winding 67 thereof is half-tappedand its output is controlled by the switch arm 68. Switch arm 68 isconnected through conductor 69 to a voltage divider 70. The slider 71thereof is connected to a twostage RC low-pass filter comprising thefirst-stage resistor 72 and capacitor 73, and the second-stage resistor74 and capacitor 76. The output terminal 77 is con- 1 which is muchsharper because of the negative feedback than the cutoff of a two-stageRC filter alone.

The control signal, after amplification by triode 79 is applied throughisolating resistors 86, 87 and 88 to three band-pass filters 89, 91 and92. These filters are constructed to pass 30, 35 and 40 C. P. 8.,respectively. Their outputs are coupled through voltage dividers 93, 94and 96 to amplifying triodes 97, 98 and 99. V The alternating potentialoutputs of these amplifying tubes are rectified by means of capacitors101, 102 and 103 and diodes 104, 106 and 107. The resultingconstant-direction potentials are integrated or smoothed in the RCintegrators consisting of resistors 108, 109 and 111, and capacitors112, 113 and 114. The resulting constant-direction smoothed potentialsat terminals 116, 117 and 118 represent the magnitudes of the original30, 35 and 40 C. P. S. control signals. These smoothed potentials arecoupled through resistors 119, 121 and 122 and applied through testswitches 123, 124 and 126 and conductors 127, 128 and 129, Fig. 2,

' to the control grid circuits of the control amplifiers at junctions131, 132 and 133, where these direct current voltages serve as gridbiases controlling the gain of the respective amplifiers. These directcurrent grid bias voltages are applied to the control amplifier controlapplied to capacitor 144 grids through isolating resistors 134, 134,136, 136', 137 and 137. These resistors also perform a second functionin cooperation with capacitors 54, 54', 56, 56', 57 and 57' of furthersmoothing the control grid directcurrent biasing voltages.

The three control amplifiers are similar. Each consists of a pair ofsharp cut-off pentodes, for example, pentodes 58 and 58'. The screengrid circuits are stiff; that is, each screened grid voltage is takenfrom the midpoint of a relatively low resistance voltage divider, suchas resistances 138 and 139, with the result that the screen grid voltageis substantially independent of screen grid current. The screen gridvoltage is slightly less than one-half of the anode voltage. The gridbias versus gain characteristic is approximately linear from minimum tomaximum gain. This circuit has the peculiar and valuable property ofhaving a sharp knee or bend at the top of the grid bias/ gaincharacteristic, so that maximum gain occurs at a selected grid bias andfurther increase of bias produces no further gain increase. Moreover,and most importantly, this limitation of gain occurs at the knee withoutany increased distortion of the output tone.

The three direct current control signals, in addition to controlling thethree control amplifiers, are added to form a sum signal for theautomatic control of switching from single-output-channel operation tovirtual stereophonic operation and vice versa. The three signals aretaken from terminals 116, 117 and 118 (Fig. 1) and are added in resistor140, 141 and 142 to form in the common conductor 143 a signalproportional to the sum of the direct-current potentials at terminals116, 117 and 118. The selection of these points for the summation,rather than an earlier point such as at the output of amplifier 79 hasseveral advantages. It is far easier to sum three direct currentsaccurately than to sum three alternating currents of ditterentfrequencies; and failure of any of the three rectifiers 104, 186 and 197causes the automatic switch to switch to singleoutput-channel operation,inclusion of these rectifiers further protecting against disruptingfailure. I

The sum potential in conductor 143 is applied to charge a capacitor 144.Since, as has been stated previously, the sum of the potentials of allthree control signals is intended to be constant, the sum potentialshould be approximately constant during virtual stereophonic operation.The potential of capacitor 144 is applied to the control grid 146 of atriode 147 having a direct current plate supply indicated by theterminal marked +B. A 60 C. P. S. source represented by terminal 148 isapplied through capacitor 149 and resistor 151 to the cathode 152. Thusthe 60 C. P. S. signal appearing between grid and cathode'is amplifiedin accordance with the tube gain as controlled by the capacitor 144potential and appears at terminal 153 with an R. M. S. amplitude changerepresenting about five times the amplitude change of the potential ofcapacitor 144. This alternating signal is connected through conductor154 to a quadrupler rectifier comprising diodes 156, 157, 158 and 159,Fig. 2,

and capacitors 161, 162, 163 and 164. This quadrupler rectifier providesa direct potential output having a gain of about fifteen times referredto changes in the potential of "capacitor 144, Fig. 1. ThisD.-C./A.-C./D.-C. amplifier system is employed rather than aconventional direct-coupled amplifier principally because it fails safe.That is, if any component should fail the output signal will fall tozero, causing the entire control unit to revert to single-output-channeloperation.

The direct voltage output of the voltage quadrupler is applied to anintegrator consisting of resistor 166,

Fig. 2, and capacitor 167. The latter is grounded through a normallyclosed relay contact 168 and relay contact armature 169. Operation ofthe armature 169 removes the ground from capacitor 167 and appliesground to a signal lamp 171, lighting it. Thus the introduction of thesum control signal switches the control unit to the virtual stereophoniccondition with a delay of about 7 seconds. However, relay 178 delaysonly two seconds in returning to normal position when the control signalis lost or reduced because capacitor 167 is out of circuit. This shortdelay is due to the integrating action of components preceding capacitor167 in the circuit.

The terminal 172 is connected to the control grid of a triode 173, wherethe signal is further amplified and applied from anode 174 throughswitch contact 176 and switch arm 177 to a relay coil 178, which isconnected in series with the source of anode positive potentialrepresented byterminal 179.

Switch arm 177 is connected mechanically to switch arms 181 and 182,comprising a three-deck four-position manually operated switch. In theupper position as shown the relay coil circuit is open. The entire powersupply is also turned ofif, as connection is broken by main switch arm182 between the llS-volt mains represented by terminal 183 and the powersupply unit 184. In the next lower switch position the power supply unitis energized and anode potential is supplied to tube 173, but coil 178is open; its relay contacts are in their normal positions, and thecontrol unit is in the single-outputchannel condition of operation. Inthe next or on switch position the circuit is ready for automaticvirtual stereophonic operation, and in the lowest or emergency onposition power is applied to relay coil 178, operating the relay andputting the control unit manually in the virtual stereophonic conditionof operation.

In the operation of the control unit with any standard motion pictureprojector, the fact that both electronic and manual gain controls in thecontrol unit are in tandem with gain controls normally provided inconjunction with the projector constitutes a condition of inherentoperating instability. In order to secure stable operation amilliammeter 186 has therefore been provided to measure the cathode/anode current of triode 173. This current is a measure of the gains andlosses inserted at the projector as, for example,- by the preamplifier13 and by any sound head output matching networks, and will change withchanges of these gains as well as of photocell sensitivity and ofexciter lamp current. The gain ahead of the control unit can thus bemaintained constant during operation by the use of milliammeter 186.

It is obviously necessary to set the gains of all three controlamplifiers to be equal with equal control signal voltages. Although thiscan be approximated by equalizing the gain control bias voltages, it ismore accurate, in that it corrects for differences among tubes, toadjust the grid control bias voltages for equal control amplifiercathode currents under the condition of maximum control signals. This isdone with the aid of taps on a four-position switch 187 to connect themilliammeter 186 to read the cathode currents of the three control tubepairs. With maximum control signals applied, voltage dividers 93, 94 and96 (Fig. 1) are adjusted to give equal cathode currents.

In adjusting the control unit, use is made of a test record containingseveral pure frequency signals alone and in combination. A four-section,three-position test switch is provided having the three contactarmatures 123, 124 and 126 (Fig. l) in the three control signal is inits midposition the control amplifier grids are returned to ground sothat tubes 58, 58, 59, 59', 60 and 60, Fig. 2, can be tested. In thethird test switch position the control amplifiers are connected fornormal operation and the transformer 19, Fig. 1, half-voltage tap isconnected. This half voltage connection is required because of thedesign of test records now available.

In employing the test record the test switch is put in its thirdposition as described. The first portion of the test record contains a1000 C. P. S. tone. Since no control signal is present, relay 178, Fig.2, remains normal for single-output-channel operation and the severalcontrols are adjusted for the desired level at loudspeaker 27. Thesecond section of the record contains only a 35 C. P. S. tone. Thisshould operate relay 178 and the control signal should pass through the35 C. P. S. channel including tubes 98, Fig. 1, 59, Fig. 2, and 59',providing opportunity to adjust this channel. The third section of thetest record contains both 1000 and 35 C. P. S. signals together at fullamplitude for the check of loudspeaker 27 loudness when actuated throughthe channel including tubes 59 and 59'. By the use of other sections ofthe test record the 30 and 40 C. P. S. chan nels are similarly adjusted.

In the operation of the control unit in connection with motion picturefilm, if the film sound track contains no control signal the relay 178remains normal. The audio signal then passes through the channelincluding relay contact armature 43, Fig. 1, normal contact 42,conductor 41, warping unit 39, Fig. 2, conductor 38, normal contact 37,contact armature 36 and power amplifier 31 to the center loudspeaker 27.If the sound track contains the 30, 3S and 40 C. P. S. control signal,relay coil 178 is energized by one or more of these signals, closing itscontacts 64, 65, 66, 46 and 168' after a delay of about'seven seconds.This delay precludes false operation of the relay during the running ofsingle-sound-channel reels by loud sounds containing frequencies in the30 to 40 C. P. S. region, since such sounds are not likely to persistfor as long as seven seconds. Armature 4-3 applies the audio signal tothe three control amplifiers; and three armatures 33, 36 and 34 connectthe three power amplifiers to their control amplifiers for virtualstereophonic operation, and armature 169 removes large capacitor 167from the circuit, so that when relay 178 is deenergized it will releaseafter'only two seconds delay. This armature 169 also energizes lamp 171as a signal that the unit is in the stereophonic condition of operation.

Thus a system is provided which will produce stereophonic sound effectsor single-output-sound channel effects, and in the event any element ofthe stereophonic control circuit fails will automatically switch thesystem to the single output condition.

What is claimed is:

1. An artificial stereophonic sound system for producingstereophonically related sound efiects from a single composite signalconsisting of an audio signal and a plurality of control signals havingdiscretely different frequencies comprising, an input circuit havingsaid composite signal impressed thereon, an audio signal channelconnected thereto, means in said audio signal channel for transmittingonly said audio signal, a plurality of control amplifiers equal innumber to the number of control signals each of which includes at leastone sharp cutofi pentode and each of which has its output coupled to aloudspeaker individual thereto, a first audio branch circuit having itsoutput connected to the control grids of all of said pentodes, a secondaudio branch circuit having its output coupled to one of saidloudspeakers at a point subsequent to the associated pentode, a controlsignal channel, means in said control signal channel for transmittingonly said control signals, a plurality of control branch circuits equalin number to the number of control signals connected to said controlsignal channel, a bandpass filter in each of said control branchcircuits each of which passes an individual and different one of saidcontrol signals, a rectifier in each of said control branch circuits forconverting the individual control signal impressed thereon into directcurrent potentials of corresponding amplitudes, means for impressingindividual ones of said direct current potentials on the control gridsof said pentodes to adjust the gain thereof, and switching means foralternatively connecting the input of said first and second audio branchcircuits to the output of said audio signal channel, said switchingmeans including means for introducing a first selected time delay inconnecting said first audio branch circuit to the output of said audiosignal channel and for introducing a second selected time delay of lessduration than said first time delay in connecting the input of saidsecond branch circuit to the output of said audio signal channel.

2. An artificial stereophonic sound system in accordance with claim 1including means for adding said direct current potentials to produce asum potential, and means for operating said switching means inaccordance with the amplitude of said sum potential.

3. An artificial stereophonic sound system in accordance with claim 1including means for adding said direct current potentials to produce asum potential, means for modulating said sum potential by an alternatingcurrent to produce a modulated sum potential, means for rectifying saidmodulated sum potential to produce a second direct current sum potentialwhose amplitude decreases to a minimum on the failure of any componentin the path between said control signal channel and said last entionedrectifying means, and means for operating said switching means toconnect the input of said first audio branch circuit to theoutput ofsaid audio signal channel in response to amplitudes of said seconddirect current sum potential above a selected level and to connect theinput of said second audio branch circuit to the output of said audiosignal channel at amplitudes of said second direct current potentialbelow said selected level. 4. An artificial stereophonic sound system inaccord ance with claim 3 including means for measuring said seconddirect current sum potential.

' 5. An artificial'stereophonic sound system for producingstereophonically related sound effects from a single composite signalconsisting of an audio signal and a plurality of control signals havingdiscretely difierent frequencies comprising, an input circuit havingsaid composite signal impressed thereon, an audio signal channelconnected thereto, a plurality of control amplifiers equal in number tothe number of control signals each of which has its output coupled toloudspeaker means individual thereto, a first audio branch circuithaving its output connected in parallel to the inputs of all of saidcontrol amplifiers, a second audio branch circuit having its outputcoupled to one of said loudspeaker means at a point subsequent to thecontrol amplifier associated therewith, a plurality of control branchcircuits equal in numher to the number of control signals each having anindividual one of said control signals impressed thereon, a rectifier ineach of said control branch circuits for converting the individualcontrol signal impressed thereon into a direct current potential ofcorresponding amplitude, means for controlling the gain of individualones of said control amplifiers by individual ones of said directcurrent potentials, means for adding said direct current potentials toproduce a sum potential, means for modulating said sum potential by analternating current to produce a modulated sum potential, means forrectifying said modulated sum potential to produce a second directcurrent potential whose amplitude decreases to a minimum on failure ofany component in the path of said control signal channel up to andincluding said last mentioned rectifying means, switching means forselecl y t finfg the inputs of said first and second audio 10 branchcircuits to the output of said audio signal channel, channel atamplitudes of said second direct current sum and means for operatingsaid switching means to conpotential below said selected level. nect theinput of said first audio branch circuit to the 7 output of said audiosignal channel in response to ampli- References Cited in the file ofthis Patent tudes of said second direct current sum potential above 5UNITED STATES PATENTS a selected level and to connect the input of saidsecond 2,714,633 Fine Aug. 2, 1955 audio branch circuit to the output ofsaid audio signal

